This invention relates in general to low thermal expansion materials and, more specifically, to low thermal expansion modified cordierite compositions especially useful in the manufacture of mirrors for high temperature applications.
Cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) has long been known to possess a low coefficient of thermal expansion (CTE), together with excellent shock resistance. Generally used as a sintered polycrystalline ceramic material, cordierite has been used in cookware and dinnerware (as described in U.S. Pat. No. 3,967,971), automobile catalytic converter substrates (U.S. Pat. No. 4,033,779), gas turbine engine components (U.S. Pat. No. 4,063,955) and other similar applications. Various additives have been suggested to improve sinterability, density, shock resistance, etc. While cordierite has a sufficiently low CTE for usefulness in these applications, the remaining thermal expansion is still a significant detriment in other applications, such as mirror substrates for high temperature uses, such as in laser systems.
Mirrors for high temperature applications are often operated in a narrow elevated temperature range. It is very difficult to manufacture a mirror at essentially room temperature, designing the optical surface so as to have acceptable optical characteristics at the higher intended operating temperature by compensating in the design for the effects of thermal expansion between the temperature at which the mirror surface is shaped and the temperature at which it is to be used. This compensation is extremely difficult with materials such as pure cordierite, which expand differently in different directions along the crystal structure.
Relatively pure cordierite is a crystalline material having relatively high refractoriness, a melting point of about 1460.degree. C. and a CTE over the range up to about 1000.degree. C. of about 16.times.10.sup.-7 C. However, the sinterability of glass powders having the stochiometry of pure corderite is poor, often resulting in porous, granular, mechanically-weak bodies when sintered at temperatures up to about 1200.degree. C. Sintering at higher temperatures causes melting and the formation of a second phase (which may be mullite, spinel, corundum, saphirine, or a combination of them in varying proportions). This uncongruent melting behavior constitutes a serious problem in the sintering of cordierite since the presence of a second phase, even in small amounts, has deleterious effects on thermal expansion. Similarly, attempts to improve sinterability or other characteristics of cordierite by adding other agents (or, simply, the use of impure raw materials) have been found to generally adversely affect thermal expansion characteristics, as described, for example, in U.S. Pat. No. 3,885,977.
Therefore, there is a continuing need for improved materials having lower thermal expansion between room and high operating temperatures while retaining the necessary sinterability, shock resistance, strength, etc., and for improvements in methods of fabricating articles from these materials.